LC Determination by Resonant Frequency Measurement using microcontroller

A well known L/C measurement circuit is pressed into service to make a bare bones measurement circuit.
Download the AVRStudio assembly source: lgm031227I.asm.
Download the AVRStudio hex file: lgm031227.hex.
Left-to-right: The 5 volt regulator, the LM393 oscillator (a 0.047 uf capacitor is mounted on the LC circuit connector), the AT90S2313, and the serial connector to the LCD module.
Recently, I have needed to measure inductances in the hundreds of microhenries to several millihenry range. Though I have a pretty good LRC meter and an excellent bridge on my workbench in Mesa, Arizona. I wanted to make these measurements in my home in Thailand. Thus I decided to put something together.

The RF Inductance meter on this website is good for low value rf inductors, but because of the way it works – putting a sharp-edged square wave through the inductor -its not suitable for inductors made with high permeability ferrites (Because of pulse shape distortion that results from high frequency losses in the core.) This meter operates at lower frequencies, and by careful selection of the resonating capacitor. The oscillator can be made to run anywhere from 100 kHz on down. This makes it possible to test near standard frequencies like 1 kHz and 400 Hz, to compare results with precision bridges.


The AT90S2313 frequency meter drives a serial terminal with a 0 to 5 volt signal. The firmware was adapted from the RS-232 Freq. Meter/Pulse Generator project on this site, and then tailored to work with the a two-line LCD (See theserial interface for Truly LCD also on this site). The serial connector has switched +8 volts to power the LCD and its interface.

Circuit Description

This is basically just an oscillator based on a comparitor and a frequency meter. The oscillator oscillates at the resonant frequency of an LC parallel tuned circuit. A really nice version of this was created by Chris Krah using an AT90S1200, including floating point math, etc. to display the L and C readings directly. I’ll post a link to Chris’ version once it is published on the web (it was posted on the AVRFreaks board).

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